The present invention relates to a system for determining the efficacy of a sterilization cycle. More specifically, the present invention relates to a system for reading fluorescence from a biological sterility indicator in order to determine the efficacy of the sterilization cycle.
The sterilization of equipment and devices is critical in some industries. For example, hospitals and other medical institutions must commonly and frequently sterilize equipment and devices used in treating patients. The particular type of sterilization cycle used to sterilize such equipment can vary based on the particular equipment or devices being sterilized and based on the particular preference of the entity performing the sterilization cycle. However, all such sterilization cycles or processes are typically designed to kill living organisms which might otherwise contaminate the equipment or devices being sterilized.
Various sterilization cycles use different methods or techniques for sterilization. For instance, such sterilization cycles may include the administration of steam, dry heat, chemicals, or radiation, to the equipment or devices being sterilized. Steam sterilization is typically efficacious when the equipment being sterilized are exposed to steam having a temperature in a range of 121-132.degree. C. The equipment being sterilized are preferably exposed to the steam sterilization for approximately three minutes at 132.degree. C., and ranging to 30 minutes at 121.degree. C. One form of chemical sterilization involves exposing the devices to be sterilized to ethylene oxide gas. The devices being sterilized are exposed to the ethylene oxide gas for approximately one hour at 65.degree. C. to approximately four hours at 30.degree. C. Dry heat sterilization typically involves exposing the devices being sterilized to temperatures in a range of approximately 180.degree. C., or higher, for at least two hours.
In many environments, the efficacy of the sterilization cycle is critical. Therefore, sterility indicators are used to determine the efficacy of the sterilization cycle.
The sterility indicators have taken a number of forms in the past. For example, biological indicators and chemical indicators are well known in the art. In conventional biological indicators, a test organism which is many times more resistent to the sterilization process than most organisms which would be present by natural contamination, is coated on a carrier and placed in a sterilizer along with the articles to be sterilized. Thus, the sterility indicator is exposed to the same sterilization cycle as the devices being sterilized. After completion of the sterilization cycle, the carrier is incubated in nutrient medium to determine whether any of the test organisms survived the sterilization procedure. Growth of a detectable number of organisms normally takes at least approximately 24 hours.
The sterility indicator is then examined to determine whether such growth has taken place. If so, such growth indicates that the sterilization cycle has not been efficacious, and it can be assumed that the devices which were subject to the sterilization cycle are not sterile.
Commercially available chemical indicators utilize chemicals which indicate sterility by color changes, or change from a solid to liquid state. One advantage to such chemical indicators is that the results are known by the end of the sterilization cycle. However, the results only indicate, for example, that a particular temperature has been reached for a certain period of time, or that ethylene oxide gas was present, during the sterilization cycle. The chemical indicators do not indicate whether conditions necessary for eliminating the organisms of interest have been achieved. Thus, the industry has shown a preference for biological indicators which use living organisms.
Another type of prior biological indicator is disclosed in Matner et al. (U.S. Pat. No. 5,418,167). Matner et al. describes a biological indicator in which a flexible polypropolene vial contains a spore strip which has a viable population of Bacillus Stearothermophilus spores. The vial also contains a growth medium which is a modified tryptic soy broth contained in a crushable glass ampule. The presence of a spore-associated enzyme, alpha-glucosidase, indicates spore growth in the biological indicator. The presence of alpha-glucosidase is measured by using a non-fluorescent substrate, 4-methylumbelliferyl-alpha-D-glucoside. The non-fluorescent substrate is converted by the active spore-associated enzyme to a fluorescent product.
If the sterilization cycle is not efficacious, both the spore and the enzyme remain active. The enzyme converts the substrate to a fluorescent product. Therefore, the fluorescence in the vial is detected, after an incubation period, to determine the efficacy of the sterilization cycle.
While Matner et al. represents a significant advancement in the art, the system for reading the biological indicator set out in Matner et al. suffers from a number of disadvantages. Matner et al.'s system is configured to read only one biological indicator at a time. Therefore, the operator places the biological indicator (BI) in a single heater, which is set to heat the biological indicator to one set temperature. The operator then sets a timer. When the timer goes off, the operator removes the biological indicator from the incubator (or heater) and places the biological indicator in the single reading cell taught by Matner, et al. The fluorescence reading is taken for the biological indicator, and an indication is provided to the operator as to whether spore growth activity is exhibited in the biological indicator (and, hence, whether the sterilization cycle to which the biological indicator has been exposed was efficacious).
Processing BIs in this way leads to a cumbersome system for tracking multiple biological indicators. First, since the system only provides a single incubator which heats to a single temperature, a separate biological indicator reading apparatus must be used in order to read different types of biological indicators (i.e., those which must be incubated at different temperatures for different time periods) Further, the user must independably chart the time when each BI was placed in the incubator, track the duration that each BI is incubated, and record the results of the reading step for each BI so as to chart the efficaciousness of each associated sterilization cycle. Also, in order to take a fluorescence reading from even a single BI, the operator must handle that BI several times. The operator must place the BI in the incubator and remove the BI when reading is desired. Then, if additional incubation or readings are desired, the operator must again place the BI in the incubator and again remove the BI for subsequent reading.
In addition, in the Matner, et al. system, the fluorescence sensor is configured to sense fluorescence emitted from one small spot on the biological indicator vessel. Thus, fluorescent activity is sensed only from a portion of the BI and not from the entire external periphery of the BI. A signal is generated based on this small amount of sensed fluorescence. This yields a sensor signal with a relatively low amplitude which must be greatly amplified in order to obtain a signal with a desirably high amplitude which can be utilized in further processing. However, the necessary amplification introduces a significant source of error in the sensor signal and also reduces the signal-to-noise ratio corresponding to the fluorescence signal.